Terahertz Fiber Research Articles

Probing terahertz metamaterials with subwavelength optical fibers

Transmission through a subwavelength terahertz fiber, which is positioned in parallel to a frequency selective surface, is studied using several finite element tools. Both the band diagram technique and the port-based scattering matrix technique are used to explain the nature of various resonances in the fiber transmission spectrum. First, we observe that spectral positions of most of the transmission peaks in the port-based simulation can be related to the positions of Van Hove singularities in the band diagram of a corresponding infinite periodic system. Moreover, spectral shape of most of the features in the fiber transmission spectrum can be explained by superposition of several Fano-type resonances. We also show that center frequencies and bandwidths of these resonances and, as a consequence, spectral shape of the resulting transmission features can be tuned by varying the fiber-metamaterial separation.

Two-wire terahertz fibers with porous dielectric support

A novel plasmonic THz fiber is described that features two metallic wires that are held in place by the porous dielectric cladding functioning as a mechanical support. This design is more convenient for practical applications than a classic two metal wire THz waveguide as it allows direct manipulations of the fiber without the risk of perturbing its core-guided mode. Not surprisingly, optical properties of such fibers are inferior to those of a classic two-wire waveguide due to the presence of lossy dielectric near an inter-wire gap. At the same time, composite fibers outperform porous fibers of the same geometry both in bandwidth of operation and in lower dispersion. Finally, by increasing cladding porosity one can consistently improve optical properties of the composite fibers.

Characterization of a Fiber Bragg Grating for Use in a THz Spectrometer

The scattering property of a terahertz fiber Bragg grating is characterized. The grating is written in subwavelength polymer fiber with a length of 160 periods. The measured full width at half maximum of the radiated beam is approximately 4 GHz, which may be extended by increasing the grating length. Such a grating shall be used in a spectrometer for the terahertz range.

Highly birefringent polymer terahertz fiber with honeycomb cladding

Two highly birefringent polymer terahertz (THz) fibers were proposed in this paper, which were formed with honeycomb cladding and some elliptical air holes in the fiber core. The losses and mode birefringence of two different fibers are investigated by finite-different time-domain method. The results show that fiber 2 can achieve both high birefringence (larger than 0.022) and low confinement loss (0.01dB/m) in a wide THz frequency range. Moreover, compared with a round solid-core fiber, guiding loss of the THz fiber caused by polymer material absorption can be reduced effectively as a part of the mode power is trapped in the air holes.

A terahertz single-polarization single-mode photonic crystal fiber with a rectangular array of micro-holes in the core region

A single-polarization single-mode (SPSM) photonic crystal fiber with a rectangular array of micro-holes in the core region was designed in the terahertz frequency range near 1THz. Based on the asymmetric arrangement of the micro-holes, birefringence between the fundamental x-polarized and y-polarized modes is introduced. A SPSM operation of the terahertz fiber can be supported due to the different mode indices of the x-polarized and y-polarized modes. The SPSM operation band is about 320GHz with a central frequency of 1THz. In addition, the proposed terahertz fiber also shows a good property of reduced propagation loss comparing with the dielectric absorption.

Label-free bacteria detection using evanescent mode of a suspended core terahertz fiber

We propose for the first time an E. coli bacteria sensor based on the evanescent field of the fundamental mode of a suspended-core terahertz fiber. The sensor is capable of E. coli detection at concentrations in the range of 10(4)-10(9) cfu/ml. The polyethylene fiber features a 150 µm core suspended by three deeply sub-wavelength bridges in the center of a 5.1 mm-diameter cladding tube. The fiber core is biofunctionalized with T4 bacteriophages which bind and eventually destroy (lyse) their bacterial target. Using environmental SEM we demonstrate that E. coli is first captured by the phages on the fiber surface. After 25 minutes, most of the bacteria is infected by phages and then destroyed with ~1 μm-size fragments remaining bound to the fiber surface. The bacteria-binding and subsequent lysis unambiguously correlate with a strong increase of the fiber absorption. This signal allows the detection and quantification of bacteria concentration. Presented bacteria detection method is label-free and it does not rely on the presence of any bacterial "fingerprint" features in the THz spectrum.

Single Mode Condition and Power Fraction of Air-Cladding Total Refractive Guided Porous Polymer Terahertz Fibers

We investigate the single-mode condition and power fraction of an air cladding total internal reflection (TIR) guided porous polymer THz fiber. It is shown that the single mode condition and a high fraction of power in air holes cannot be simultaneously met in porous fibers. Employing the V parameter and the Lorentz—Lorenz formula, we analyze this problem theoretically.

Characterization of a microstructured Zeonex terahertz fiber

We report on the characterization of Zeonex polymer in the terahertz (THz) region and the fabrication and characterization of a microstructured polymer fiber made of Zeonex. We demonstrate single-mode propagation with highly efficient coupling into the fiber close to the theoretical limit of 80% using specially designed lenses. The THz time domain measurements allow the loss and dispersion properties of the fiber to be determined, showing that for the current fiber the losses are only caused by the material loss. The phase refractive index is calculated from experimental data and compared to values predicted by scalar and vectorial simulations. Results for the dispersion parameter β2 for a THz microstructured fiber are presented for the first time to our knowledge.

Ultrahigh birefringent polymer terahertz fiber based on a near-tie unit

A novel highly birefringent polymer terahertz (THz) fiber based on a near-tie unit,which is formed by three tangent circular air holes, is proposed in this paper.The introduction of near-tie units in the fiber core can enhance asymmetry torealize high mode birefringence. The dependence of the birefringence on severalparameters (radii of the air holes, unit-to-unit spacing lattice constant and latticearrangement), the power distribution of the mode power and the relative absorptionloss are investigated with a full-vector finite element method (FEM). Simulationresults show that the polymer THz fiber exhibits high birefringence on a level of10 − 2 over a wide frequency range, and an extremely large birefringence (≈5.11 × 10 − 2 atf = 1.2 THz) is obtained in a triangular lattice THz fiber. Moreover, compared with a roundsolid-core fiber, the THz fiber guiding loss caused by polymer material absorption canbe reduced effectively as a part of the mode power is trapped in the air holes.

Direct probing of evanescent field for characterization of porous terahertz fibers

We develop a technique based on a micromachined photoconductive probe-tip to characterize a terahertz (THz) porous fiber. Losses less than 0.08 cm−1 are measured in the frequency range from 0.2 to 0.35 THz, with the minimum of 0.003 cm−1 at 0.24 THz. Normalized group velocity greater than 0.8, which corresponds to dispersion values in between −1.3 and −0.5 ps/m/μm for 0.2<f<0.35 THz are obtained. Moreover, we directly measure the evanescent electric field as a function of frequency. Good agreement between the measured curves and expected theoretical values indicates the low invasiveness of the applied probe-tip.

A porous terahertz fiber with randomly distributed air holes

A random porous fiber for transporting terahertz (THz) waves is proposed in this paper. The fiber, which to the best of our knowledge has not been investigated previously, has its core composed by randomly distributed air holes. A numerical study was carried out near 1 THz, showing that the new fiber possesses some improved fiber properties compared to the regular periodic porous fiber: the power fraction confined in the core is higher, the dispersion becomes lower, and more importantly a flatter dispersion curve appears within the frequency range of 0.8 THz to 1.4 THz.

Highly Birefringent Terahertz Fibers Based on Super-Cell Structure

A kind of novel highly birefringent terahertz (THz) fibers based on super-cell structure, which has the advantage of minimizing the structural distortion during fabrication is proposed. The dielectric material (such as polymer) with airholes and the surrounding air are used as the fiber core and the fiber cladding, respectively. Uniform airholes with the diameter that is much smaller than the operation wavelength are used to form the basic cell. Two typical highly birefringent THz fibers with cross sections of the diamond-shaped and the dumbbell-shaped cells are introduced and investigated, respectively. High birefringence up to the order of 0.01 is achieved, and the power distributions of the modal power and the relative absorption loss of the proposed THz fibers are also investigated. The proposal for the THz fibers based on super-cell structure indicates a method to achieve unique property as the THz fibers with complex cross section.

Fiber-based swept-source terahertz radar

We demonstrate an all-terahertz swept-source imaging radar operated at room temperature by using terahertz fibers for radiation delivery and with a terahertz-fiber directional coupler acting as a Michelson interferometer. By taking advantage of the high water reflection contrast in the low terahertz regime and by electrically sweeping at a high speed a terahertz source combined with a fast rotating mirror, we obtained the living object's distance information with a high image frame rate. Our experiment showed that this fiber-based swept-source terahertz radar could be used in real time to locate concealed moving live objects with high stability.

Single-mode propagation in triangular tube lattice hollow-core terahertz fibers

The mode properties of hollow-core fibers with a cladding formed by a periodic arrangement of dielectric tubes in a triangular lattice for THz applications are numerically investigated. The fiber supports a high number of modes. Effectively single-mode operation can be obtain by reducing the core size, but at the expense of high fundamental mode propagation loss. Single-mode propagation can be obtained by exciting the fiber with a linear polarized gaussian beam with proper spot size.

Squeezed lattice elliptical-hole terahertz fiber with high birefringence

A highly birefringent elliptical-hole terahertz (THz) fiber with a squeezed lattice is proposed. This THz fiber is formed through regular linear deformation of a circular porous fiber by squeezing it in one direction, which may be fabricated with the existing fabrication technology. By numerical analysis we show that, with a moderate amount of deformation, the proposed THz fiber can exhibit high birefringence on a level of 10(-2) over a wide THz frequency range. And the THz fiber guiding loss caused by material absorption can be reduced effectively, since a dominant fraction of modal power is distributed in the airholes inside the dielectric material.

Polarization Maintaining Air-Core Bandgap Fibers for Terahertz Wave Guiding

A polarization-maintaining air-core bandgap polymer fiber is proposed for low-loss terahertz (THz) wave guiding. The polarized guided modes are confined in an asymmetric core by a bandgap of periodic arrangement of square holes with round corners in the cladding. The guiding properties, including transmission bandwidth, numerical aperture, phase-index birefringence, modal absorption loss, and bend loss, are systematically investigated. The influence of background polymers on guiding properties is demonstrated in detail. Numerical simulations reveal that, while maintaining a relatively high phase-index birefringence (of the order of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> ), the THz fiber shows a significant suppression of absorption loss of the background polymers (by a factor of more than 25) due to the bandgap effect that repels the modal power from the absorbent polymers. The proposed THz fiber has potential for guiding intense THz waves for polarization-sensitive applications.

Low-loss air-core polarization maintaining terahertz fiber

We propose a low-loss air-core polarization maintaining polymer fiber for terahertz (THz) wave guiding. The periodic arrangement of square holes with round corners in the cladding offers a bandgap effect for mode guiding. Numerical simulations show that the bandgap effect repels the modal power from the absorbent background polymers, resulting in a significant suppression of absorption loss of the polymers by a factor of more than 25. The phase-index birefringence of the proposed THz fiber is in the order of 10(-3).

Low loss porous terahertz fibers containing multiple subwavelength holes

We propose a porous polymer terahertz fiber with a core composed of a hexagonal array of subwavelength air holes. Numerical simulations show that the larger part of guided power propagates inside the air holes within the fiber core, resulting in suppression of the bulk absorption losses of the core material by a factor of ∼10–20. Confinement of terahertz power in the subwavelength holes greatly reduces effective refractive index of the guided mode but not as much as to considerably increase modal radiation losses due to bending. As a result, tight bends of several centimeter bending radii can be tolerated.

Field enhancement and power distribution characteristics of subwavelength-diameter terahertz hollow optical fiber

Fields in subwavelength-diameter terahertz hollow optical fiber (STHOF) can be intensified by large discontinuity of the electric field at high index contrast interfaces. The influences of fiber geometry and refractive index of the dielectric region on the fiber characteristics, such as power distribution, enhancement factor, have been discussed in detail. By appropriate design, the intensity in the central region of STHOF may be enhanced by a factor of greater than 1.5 compared with subwavelength-diameter terahertz fiber without the central hole and the loss can be reduced. For its compact structure and simple fabrication process, the fiber may be very useful in many miniaturized high performance and novel terahertz photonic devices.

Flexible terahertz fiber optics with low bend-induced losses

We developed a metal hollow waveguide as a flexible delivery medium of terahertz waves. Theoretical evaluation shows that a metal waveguide mainly supports TE modes and that the TE11 mode shows a high coupling coefficient to linearly polarized light. Also the TE11 mode brings less mode conversion to high-order modes than the TE01 mode giving the lowest loss. We measured transmission losses in the terahertz region of hollow waveguides with inner silver coating; the losses were ∼7.5-8dB/m at the wavelengths from 190-250μm for waveguides with an inner diameter of 1mm. These losses coincide well with theoretical ones, and this shows that in these waveguides, the TE11 mode is dominant when a linearly polarized beam is launched into them. The waveguides are flexible because we use a thin-wall glass capillary as the base tubing. Our experiment revealed low additional losses due to bending even when complicated bends were applied to the waveguides. The metal-coated hollow fiber with an inner diameter of 1mm is sufficiently small and flexible for use in endoscopic applications.